As a power source for driving a cordless electric tool, used is a secondary battery having a capacity relatively increased such as a nickel hydrogen battery or a NiCd battery. As the secondary battery having a capacity more increased and a weight more decreased, a lithium battery including a lithium ion battery has been progressively practically used.
The lithium battery has features that the nominal voltage of the cell is about three times as high as that of the nickel hydrogen battery or the NiCd battery widely put to practical use, and the battery is compact and light. Further, the lithium battery has features that the battery has good discharge efficiency, the battery can be discharged even in an environment of relatively low temperature and the battery can obtain a stable voltage in a wide range of temperature.
The lithium battery (battery pack) is ordinarily charged under a constant current/constant voltage control. However, it is generally known that to overcharge the battery under the constant current/constant voltage control is undesirable in view of the characteristics or the life of the battery. Accordingly, for instance, as disclosed in JP-A-2-192670, a method is proposed that while a battery is charged under a constant voltage, the charging current of a charging power circuit is detected, and when the charging current is lowered to a value not higher than a prescribed value, the battery is decided to be fully charged to generate a charging stop signal, interrupt a charging path of the charging power circuit and finish a charging operation.
Further, when the battery is ordinarily charged by a relatively large current, the battery can be charged in a short time. However, when a battery temperature at the time of charging the battery is low, the cycle life of the battery is short. Accordingly, a battery charger is proposed in which in the beginning of a charging operation, a battery is charged by a relatively small current, and when the battery temperature rises to a prescribed value during the charging operation, a charging current is increased stepwise to charge the battery in a short time. For instance, such a technique is disclosed in JP-A-4-340330.